The present invention relates to a method of forming a liquid crystal polymer layer in which the molecules have a predetermined orientation. It relates particularly, though not exclusively, to methods of forming unsupported films less than 6 microns thick for use as retardation plates, for example in fiber optic systems.
The polarization state of light can be varied by passing it through a retardation plate. For example, when linearly polarized electromagnetic radiation having a wavelength of 1550 nm is passed through a retardation plate having a retardation of 775 nm it is converted to circularly polarized electromagnetic radiation. The process also works in reverse, so that when circularly polarized electromagnetic radiation having a wavelength of 1550 nm is passed through a retardation plate having a retardation of 775 nm, linearly polarized electromagnetic radiation is produced. Such a plate is called a half wave plate. Such plates are often made from stretched polycarbonate or polyvinyl alcohol (PVA) films. However, due to the low birefringence of such films they have to be of the order of 100 to 300 microns thick, depending upon the wavelength of the electromagnetic radiation used.
For some applications, for example for fiber optic applications in the telecomm industry, very thin films are required, which can be made from aligned liquid crystal polymers. These films can be made by traditional coating techniques on glass substrates, but when this is done, the film often adheres strongly to the substrate, such that when removed, the liquid crystal polymer film stretches, changing its retardation. Thus they do not readily release from the substrate and cannot be mechanically removed due to their fragile nature (often only a few microns thick). In some applications, the transmission losses associated with inserting the glass substrate (which is typically greater than 50 microns in thickness) between optical fibers make the use of films supported by the substrate unattractive.
The present invention relates to a method of forming a liquid crystal polymer layer. The method includes the steps of: coating a substrate with a release layer; coating the release layer with an alignment layer capable of aligning a liquid crystal polymer layer; coating the alignment layer with a layer of a liquid crystal polymer material, thereby forming a liquid crystal polymer layer in which the molecules have a predetermined orientation; solidifying the layer of liquid crystal polymer; and dissolving the release layer in a liquid which does not harm the solidified liquid crystal polymer layer to form a self supporting film. The release layer and alignment layer can be combined into a single layer, such as for example a layer of polyvinyl alcohol which may be dissolved in water. Thicker or multi-layer films can be fabricated by using the first solidified layer as an alignment layer for a subsequently deposited liquid crystal polymer layer.
The present invention also relates to a retardation plate made using the claimed method.